1. Field of the Invention
This invention pertains to load sensing devices, and more particularly, to sensing devices used to detect an overload condition on a gear used to rotate an upper structure mounted on a vehicle.
2. Background of the Invention
The problems associated with cranes and other types of lifting devices mounted on vehicles have long been recognized. In particular, problems associated with overloading by swinging the boom to the sides of the vehicle and in pulling articles with the boom can result in failure of the boom. Typical situations in which an overload on the boom may occur are when the vehicle is on sloped terrain while the boom is being articulated, when the boom being articulated strikes an object to the side of the vehicle, and when the boom, extended to the side of the vehicle, is used to pull a load, either by forward movement of the vehicle or by continued articulation of the boom. Once the boom has been overloaded, the operator typically does not have sufficient time to shutoff power to the lifting device or to the gear that rotates the boom to prevent the dangerous conditions.
Lifting devices which include mechanical means for shutting off the power to the lifting device and/or sending a signal to the operator in the event of an emergency are disclosed in U.S. Pat. Nos. 4,625,946 and 2,300,343 and Great Britain Patent No. 950,003. A More recent side load sensing device which includes means for shutting off the power is disclosed in U.S. Pat. No. 5,163,570, issued to Mundis et al on Nov. 17, 1992. The Mundis et al patent operates on a hydraulic principle; when the worm is subject to a thrust along its axis, in either direction, an increase in hydraulic fluid pressure sends a signal and/or terminates input power to the worm. Other patents of interest are U.S. Pat. Nos. 5,597,080; 5,557,526 and 5,359,516.